Lithium Niobate Vertical Cavity Electro-Optic Modulator

TL;DR

The paper tackles the need for compact free-space electro-optic modulators by realizing a vertical-cavity EOM in which a LiNbO3 membrane is sandwiched between two photonic-crystal mirrors, forming a defect-mode cavity that shifts under the Pockels effect. A refractive-index change $|\Delta n_e| = \frac{1}{2} n_e^3 \gamma_{33} \frac{V_{pp}}{d}$ enables strong transmission modulation of z-polarized light near 787–800 nm, driven by integrated electrodes aligned with the optic axis. The device demonstrates a Q-factor above 600 and a resonance linewidth of about 1.3 nm, with a maximum modulation depth of 0.43 at $V_{pp} = \pm 50$ V and a 3 dB bandwidth of ~5 MHz (intrinsic bandwidth anticipated in the GHz). This approach yields a compact, free-space compatible platform with potential applications in ranging, dynamic holography, and beam steering.

Abstract

Electro-optic modulators (EOMs) are vital for optical imaging and information processing, with free-space devices enabling LiDAR and beam control. Lithium niobate (LN), powered by the strong Pockels effect and scalable LN-on-insulator (LNOI) platform, has become a leading material for high-performance EOMs. Here we realize a vertical-cavity EOM in which an LN membrane is sandwiched between two photonic crystal (PhC) mirrors with integrated electrodes. The cavity supports sharp defect-mode resonances that shift efficiently under the Pockels effect, enabling strong modulation of transmission. Experiments show a depth of 43 % at 50 V and a bandwidth of 5 MHz. This architecture combines free-space compatibility with fabrication simplicity, opening new routes to compact electro-optic platforms for ranging, holography, and beam steering.

Figures (4)

• Figure 1: Schematic of the LN Vertical-Cavity EOM. The device consists of a thin LN membrane vertically sandwiched between two photonic crystal (PhC) mirrors, each composed of alternating TiO$_2$ and SiO$_2$ layers. The coordinate system is aligned with the principal axes of the LN crystal, with the $z$-axis along the optic axis. Two gold electrodes are embedded at the interface between the LN membrane and the bottom PhC. This vertical-cavity configuration supports a series of resonances in the transmission spectrum. A square-wave voltage with peak-to-peak amplitude $V_{pp}$ is applied to the electrodes, while a $z$-polarized light beam impinges normally on the modulator surface, producing a modulated optical output.
• Figure 2: Band structure engineering and simulated performance of the vertical cavity EOM. (a) Top: Simulated photonic band structure of the PhC, showing a bandgap centered near a normalized frequency of 0.28. Bottom: The 1 $\mu$m LN defect introduces cavity modes around 700, 800 and 920 nm. (b) Transmission spectra around 800 nm under different modulation voltages: dashed blue, +50 V; solid black, 0 V; dashed red, -50 V. (c) Simulated modulation depth ($M$) corresponding to the $\pm$50 V voltage switching.
• Figure 3: Fabrication of the EOM. (a) The fabrication process: starting from an LNOI wafer with an $x$-cut LN film bonded to a silicon substrate via a SiO$_2$ buffer layer, the LN film was released using HF etching. Gold electrodes with a 10 $\mu$m pitch were patterned on a fused silica substrate pre-coated with PhC1 (alternating TiO$_2$/SiO$_2$ layers). The LN film was then transferred and aligned onto the electrodes, followed by deposition of PhC2 on the LN to form the vertical cavity. (b) Left: packaged vertical EOM. Bottom right: optical microscopy of the modulator surface. Top right: SEM image showing the detailed cross-section of the LN vertical cavity. (c) Experimental transmission spectra (red curves) under normally incident $z$-polarized light, compared with simulations (black dashed curves) incorporating fabrication-induced optical losses.
• Figure 4: Experimental characterization of the vertical cavity EOM. (a) Schematic of the optical setup. The tunable laser is directed through a polarizer (Pol) and a micro-focusing system with dual-aperture filtering to illuminate the modulator, which is simultaneously driven by a synchronous electrical signal. Spectral characteristics are achieved through the combined use of a supercontinuum source (SC), spectrometer (SM), and beam splitter (BS). A bypass photodetector (PD1) captures the pulse signal and couples with the signal source to generate a synchronous modulation drive signal. High-level and low-level pulse modulation is achieved by adjusting the phase delay. The end photodetector (PD2) collects the output signal, and the modulation results are analyzed via a phase-locked amplifier. (Illustration: Pulse in-phase operation timing diagram). (b) Transmission spectrum of the sample measured by the supercontinuum light source spectral testing system. Blue circles: experimental data; red curves: Fano-fitted spectra, yielding Q-values of 611. (c) Measured modulation depth ($M$) from 0 to ±50 V in 10 V steps. Dots: experimental data; dotted lines: visual guide. (d) Optical power modulation under differential frequency excitation, showing representative intensity profiles at maximum (top inset) and minimum (bottom inset) modulation states. (e) Modulation bandwidth of devices under optical power modulation with multiple-frequency differential excitation.